Add global CMake setup for examples

When working from the checked out clone of the exercises it's very convienent to have a top level CMakeLists.txt file, so that all the example code can be built one-shot from the same environment.

This requires renaming each executable target to have the day as part of it name - it's an error to have multiple CMake targets with the same name.

Also add a .gitignore file that covers most of the ephemeral stuff that crops up working with the project with common editor/IDE setups.